This invention relates to guidance of rotating vehicles, GPS guidance of artillery shells, and more specifically interference-aided navigation and temporal beam forming using interference signals.
Spinning vehicles such as artillery shells, missiles, bombs, and uninhabited aerial vehicles (UAV) are using navigation and guidance techniques to accurately reach their designated targets. Techniques in use include inertial navigation, global positioning system (GPS) guidance, and magnetic field sensing.
In many applications, such as artillery shell or missile guidance, the vehicle to be guided is spinning rapidly. The small size and high rotation rate of the vehicle greatly complicate traditional navigation enhancement techniques such as inertial aiding and multi-antenna beam forming as disclosed in co-pending application 01CR040/KE Ser. No. 10/005,237 filed Dec. 5, 2001. The small size of an artillery shell or fuse does not permit the use of large antenna arrays or complex RF electronics. Inertial navigation is very difficult to implement because even very small scale factor errors and instabilities lead to large and rapidly increasing attitude errors. For example, a shell spinning at 250 Hz is turning at 90,000xc2x0/s. A scale factor error of only 11 ppm will cause an attitude error rate of 1xc2x0/s. The small gyros that could be packaged into an artillery round or fuse do not have the precision to maintain an accurate attitude estimate at these rotation rates. Traditionally, guided shell designers have been forced to resort to mechanical de-spinning systems that add cost, size, and complexity. Shells that are not de-spun have utilized omnidirectional antennas with limited jamming immunity for GPS signal reception, and no inertial aiding.
U.S. Pat. No. 6,208,936 describes some of the difficulties encountered in creating an effective navigation system for a rapidly spinning vehicle such as an artillery shell. A system is disclosed that utilizes a magnetic field sensor for tracking the rotation angle of the vehicle and a system for computationally de-spinning the vehicle to greatly simplify calculation and improve accuracy of the navigation solution. U.S. Pat. No. 6,163,021 also discloses utilizing a magnetic field sensor to de-spin the body-axis frame measurements and, in addition, using accelerometers to measure the Coriolis accelerations due to rotation. The need for gyros and their associated rate range and scale factor limitations is eliminated. The technique of utilizing accelerometers to measure rotation rate, in a spinning frame, is well known and disclosed in U.S. Pat. No. 4,520,669.
Use of a magnetic sensor for roll determination in a spinning vehicle can be effective under the correct circumstances. However, this approach requires the addition of a magnetic sensor and performance can be dependent on the magnetic properties of the vehicle and its electrical systems as well as its position on the earth and the magnetic environment. For example, near the equator a vehicle traveling approximately due north or south will have difficulty in determining its rotation angle.
Cited co-pending application Ser. No. 09/879,392 filed Jun. 12, 2001 describes an advanced spinning-vehicle navigation (AVSN) system utilizing GPS or similar navigation signals to determine the rotation angle of the vehicle. This approach offers significant improvement in performance and robustness under interference once the navigation signals are acquired. However, the application does not describe any technique to enhance initial acquisition under high levels of jamming or interference.
Artillery navigation systems have utilized omnidirectional antennas to reduce the phase and amplitude modulation as a function of projectile roll angle. This approach has poor performance under conditions of interference and jamming because the GPS signal and interference source are continuously received at the same relative gain. The use of a directional antenna will tend to improve interference immunity because the GPS receiver""s AGC control will tend to reduce gain when the antenna is pointed toward the jamming source, and tend to increase the gain when the antenna is pointed away from the interference. Unfortunately the directional antennas tend to produce large phase modulations with rotation, and performance is not fully optimized because the received signal is still being processed during the times when the antenna is pointed toward the jammer and signal to noise ratios may be close to zero.
The phase and amplitude modulation, combined with any external interference and jamming signal sources can make it difficult to acquire and track low-level navigation signals such as GPS. A practical, cost-effective, technique is needed for spinning vehicles to acquire and track low-level signals in the presence of large interference or jamming signals.
An interference-aided signal acquisition and tracking system with temporal beam forming for a rotating vehicle to enhance signal to noise ratio is disclosed. The system includes an antenna that receives interference signals and desired navigation signals. A RF processing function connected to the antenna processes the received interference signals and the desired navigation signals into IF signals. An A/D converter connected to the RF processing function digitizes the interference signals and desired navigation signals to provide a digitized IF signal. A tracking filter tracks amplitude variations of the interference signals and provides a rotation angle estimate signal of the rotating vehicle. A signal amplitude modulation function connected to the tracking filter and the A/D converter varies the gain of the IF signal to enhance the signal to noise ratio.
An intensity detector connected to the RF processing function and the tracking filter may be used for determining the level of the interfering signal and for providing an input modulation signal to the tracking filter. An AGC loop connected to the A/D converter and to the tracking filter may provide an input modulation signal to the tracking filter.
The tracking filter may further comprise a first mixer for receiving the input modulation signal from the AGC loop and mixing the input modulation signal with a first demodulation signal. A first filter in the tracking filter amplifies and filters the first mixer output signal to provide a phase error feedback signal. A tracking servo receives the phase error feedback signal, provides the first demodulation signal and provides the rotation angle estimate signal.
The signal amplitude modulation function comprises a signal modulation controller that receives the rotational angle estimate from the tracking filter and provides a modulation control signal. A gain modulation function increases gain when the antenna is pointed toward a navigation signal source and reduces gain when the antenna is pointed toward an interfering signal source to enhance signal to noise ratio.
The interference-aided signal acquisition and tracking system for a rotating vehicle may further comprise a second antenna for receiving interference signals and desired navigation signals, a second RF processing function connected to the second antenna to process the received interference signals and the desired navigation signals and for cross-feeding signals to the first RF processing function, and a second A/D converter connected to the second RF processing function to digitize the interference signals and desired navigation signals and to provide a second IF signal to further enhance performance.
It is an object of the present invention to provide acquisition of navigation and other signals in spinning vehicles.
It is an object of the present invention to provide a system that acquires and tracks low-level signals in the presence of large interference and jamming signals.
It is a feature of the present inventions to utilize the amplitude modulation of any present interference signal to aid in the determination of vehicle rotation rate to aid in the acquisition of navigation and other signals.
It is a feature of the present invention obtain roll angle estimates using interference or jamming signals from an interference tracking loop to improve signal to noise ratios.
It is an advantage of the present invention to provide significant signal to noise ratio improvements over omnidirectional antennas.
It is an advantage of the present invention to correct phase and optimize gain of a GPS signal away from a jammer by using the jammer signal.